Air spacers in transistors and methods forming same

ABSTRACT

A method includes forming a gate stack over a semiconductor region, and forming a first gate spacer on a sidewall of the gate stack. The first gate spacer includes an inner sidewall spacer, and a dummy spacer portion on an outer side of the inner sidewall spacer. The method further includes removing the dummy spacer portion to form a trench, and forming a dielectric layer to seal a portion of the trench as an air gap. The air gap and the inner sidewall spacer in combination form a second gate spacer. A source/drain region is formed to have a portion on an outer side of the second gate spacer.

BACKGROUND

Technological advances in Integrated Circuit (IC) materials and designhave produced generations of ICs, with each generation having smallerand more complex circuits than the previous generations. In the courseof IC evolution, functional density (for example, the number ofinterconnected devices per chip area) has generally increased whilegeometry sizes have decreased. This scaling down process providesbenefits by increasing production efficiency and lowering associatedcosts.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,Fin Field-Effect Transistors (FinFETs) have been introduced to replaceplanar transistors. The structures of FinFETs and methods of fabricatingFinFETs are being developed.

The formation of FinFETs typically involves forming semiconductor fins,implanting the semiconductor fins to form well regions, forming dummygate electrodes on the semiconductor fins, etching some portions of thesemiconductor fins, and performing an epitaxy to regrow source/drainregions.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 4, 5A, 5B, and FIGS. 6 through 9 are perspective viewsof intermediate stages in the formation of a Fin Field-Effect Transistor(FinFET) including air gaps in gate spacers in accordance with someembodiments.

FIGS. 10 through 17, 18A, 18B, 19A, and 19B are cross-sectional views ofintermediate stages in the formation of air gaps in gate spacers beforesource/drain epitaxy in accordance with some embodiments.

FIGS. 20 through 25, 26A, and 26B are cross-sectional views ofintermediate stages in the formation of air gaps in gate spacers afterthe formation of replacement gates in accordance with some embodiments.

FIGS. 27 through 34, 35A, and 35B are cross-sectional views ofintermediate stages in the formation of air gaps in gate spacers afterthe formation of source/drain contact plugs in accordance with someembodiments.

FIG. 36 illustrates a top view of a transistor having air gaps in gatespacers in accordance with some embodiments.

FIG. 37 illustrate various shapes of air gaps in gate spacers inaccordance with some embodiments.

FIG. 38 illustrates a process flow for forming a transistor and contactplugs in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

Transistor and the methods of forming the same are provided inaccordance with various exemplary embodiments. The intermediate stagesof forming the transistors are illustrated in accordance with someembodiments. Some variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements. In theillustrated exemplary embodiments, the formation of Fin Field-EffectTransistors (FinFETs) is used as an example to explain the concept ofthe present disclosure. Planar transistors may also adopt the concept ofthe present disclosure.

FIGS. 1 through 9 illustrate the perspective views of intermediatestages in the formation of air gaps in gate spacers in accordance withsome embodiments of the present disclosure. The steps shown in FIGS. 1through 9 are also reflected schematically in the process flow 200 asshown in FIG. 38.

FIG. 1 illustrates a perspective view of an initial structure. Theinitial structure includes wafer 10, which further includes substrate20. Substrate 20 may be a semiconductor substrate, which may be asilicon substrate, a silicon germanium substrate, or a substrate formedof other semiconductor materials. Substrate 20 may be doped with ap-type or an n-type impurity. Isolation regions 22 such as ShallowTrench Isolation (STI) regions may be formed to extend from a topsurface of substrate 20 into substrate 20. The portions of substrate 20between neighboring STI regions 22 are referred to as semiconductorstrips 24. The top surfaces of semiconductor strips 24 and the topsurfaces of STI regions 22 may be substantially level with each other.In accordance with some embodiments of the present disclosure,semiconductor strips 24 are parts of the original substrate 20, andhence the material of semiconductor strips 24 is the same as that ofsubstrate 20. In accordance with alternative embodiments of the presentdisclosure, semiconductor strips 24 are replacement strips formed byetching the portions of substrate 20 between STI regions 22 to formrecesses, and performing an epitaxy to regrow another semiconductormaterial in the recesses. Accordingly, semiconductor strips 24 areformed of a semiconductor material different from that of substrate 20.In accordance with some exemplary embodiments, semiconductor strips 24are formed of silicon germanium, silicon carbon, or a III-V compoundsemiconductor material.

STI regions 22 may include a liner oxide (not shown), which may be athermal oxide formed through a thermal oxidation of a surface layer ofsubstrate 20. The liner oxide may also be a deposited silicon oxidelayer formed using, for example, Atomic Layer Deposition (ALD),High-Density Plasma Chemical Vapor Deposition (HDPCVD), or ChemicalVapor Deposition (CVD). STI regions 22 may also include a dielectricmaterial over the liner oxide, wherein the dielectric material may beformed using Flowable Chemical Vapor Deposition (FCVD), spin-on, or thelike.

Referring to FIG. 2, STI regions 22 are recessed, so that the topportions of semiconductor strips 24 protrude higher than the topsurfaces 22A of the remaining portions of STI regions 22 to formprotruding fins 24′. The respective step is illustrated as step 202 inthe process flow 200 as shown in FIG. 38. The etching may be performedusing a dry etching process, wherein HF₃ and NH₃ are used as the etchinggases. During the etching process, plasma may be generated. Argon mayalso be included. In accordance with alternative embodiments of thepresent disclosure, the recessing of STI regions 22 is performed using awet etch process. The etching chemical may include HF, for example.

In above-illustrated exemplary embodiments, the fins may be patterned byany suitable method. For example, the fins may be patterned using one ormore photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers, or mandrels, may then be used to pattern thefins.

Referring to FIG. 3, dummy gate stacks 30 are formed on the top surfacesand the sidewalls of (protruding) fins 24′. The respective step isillustrated as step 204 in the process flow 200 as shown in FIG. 38.Dummy gate stacks 30 may include dummy gate dielectrics 32 and dummygate electrodes 34 over dummy gate dielectrics 32. Dummy gate electrodes34 may be formed, for example, using polysilicon, and other materialsmay also be used. Each of dummy gate stacks 30 may also include one (ora plurality of) hard mask layer 36 over the respective dummy gateelectrodes 34. Hard mask layers 36 may be formed of silicon nitride,silicon oxide, silicon carbo-nitride, or multi-layers thereof. Dummygate stacks 30 may cross over a single one or a plurality of protrudingfins 24′ and/or STI regions 22. Dummy gate stacks 30 also havelengthwise directions perpendicular to the lengthwise directions ofprotruding fins 24′.

Next, gate spacers 38 are formed on the sidewalls of dummy gate stacks30. In accordance with some embodiments of the present disclosure, gatespacers 38 are formed of a dielectric material such as silicon nitride,silicon carbo-nitride (SiCN), silicon oxy-carbo-nitride (SiOCN), or thelike, and may have a single-layer structure or a multi-layer structureincluding a plurality of dielectric layers.

In accordance with some embodiments of the present disclosure, air gaps39 are formed in gate spacers 38. Air gaps 39 may be filled with air, ormay be vacuumed or substantially vacuumed to have a pressure lower thanone atmosphere. The processes for forming the corresponding air gaps 39are shown in detail in FIGS. 10 through 18, 19A, and 19B, and discussedaccordingly. In accordance with alternative embodiments of the presentdisclosure, air gaps 39 are not formed in gate spacers 38 at this time.Rather, air gaps 39 are formed in subsequent processes. Accordingly airgaps 39 are illustrated as dashed to indicate they may or may not existat this stage.

In accordance with some embodiments of the present disclosure, anetching step (referred to as source/drain recessing hereinafter) isperformed to etch the portions of protruding fins 24′ that are notcovered by dummy gate stack 30 and gate spacers 38, resulting in thestructure shown in FIG. 4. The recessing may be anisotropic, and hencethe portions of fins 24′ directly underlying dummy gate stacks 30 andgate spacers 38 are protected, and are not etched. The top surfaces ofthe recessed semiconductor strips 24 may be lower than the top surfaces22A of STI regions 22 in accordance with some embodiments. Recesses 40are accordingly formed between STI regions 22. Recesses 40 are locatedon the opposite sides of dummy gate stacks 30.

Next, epitaxy regions (source/drain regions) 42 are formed byselectively growing a semiconductor material in recesses 40, resultingin the structure in FIG. 5A. The respective step is illustrated as step206 in the process flow 200 as shown in FIG. 38. In accordance with someexemplary embodiments, epitaxy regions 42 include silicon germanium orsilicon. Depending on whether the resulting FinFET is a p-type FinFET oran n-type FinFET, a p-type or an n-type impurity may be in-situ dopedwith the proceeding of the epitaxy. For example, when the resultingFinFET is a p-type FinFET, silicon germanium boron (SiGeB) may be grown.Conversely, when the resulting FinFET is an n-type FinFET, siliconphosphorous (SiP) or silicon carbon phosphorous (SiCP) may be grown. Inaccordance with alternative embodiments of the present disclosure,epitaxy regions 42 are formed of a III-V compound semiconductor such asGaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP, combinationsthereof, or multi-layers thereof. After epitaxy regions 42 fully fillrecesses 40, epitaxy regions 42 start expanding horizontally, and facetsmay be formed.

After the epitaxy step, epitaxy regions 42 may be further implanted witha p-type or an n-type impurity to form source and drain regions, whichare also denoted using reference numeral 42. In accordance withalternative embodiments of the present disclosure, the implantation stepis skipped when epitaxy regions 42 are in-situ doped with the p-type orn-type impurity during the epitaxy to form source/drain regions. Epitaxysource/drain regions 42 include lower portions that are formed in STIregions 22, and upper portions that are formed over the top surfaces ofSTI regions 22.

FIG. 5B illustrates the formation of source/drain regions 42 inaccordance with alternative embodiments of the present disclosure. Inaccordance with these embodiments, the protruding fins 24′ as shown inFIG. 3 are not recessed, and epitaxy regions 41 are grown on protrudingfins 24′. The material of epitaxy regions 41 may be similar to thematerial of the epitaxy semiconductor material 42 as shown in FIG. 5A,depending on whether the resulting FinFET is a p-type or an n-typeFinFET. Accordingly, source/drains 42 include protruding fins 24′ andthe epitaxy regions 41. An implantation may be performed to implant ann-type impurity or a p-type impurity.

FIG. 6 illustrates a perspective view of the structure after theformation of Contact Etch Stop Layer (CESL) 46 and Inter-LayerDielectric (ILD) 48. The respective step is illustrated as step 208 inthe process flow 200 as shown in FIG. 38. CESL 46 may be formed ofsilicon nitride, silicon carbo-nitride, or the like. In accordance withsome embodiments of the present disclosure, CESL 46 is formed using aconformal deposition method such as ALD or CVD, for example. ILD 48 mayinclude a dielectric material formed using, for example, FCVD, spin-oncoating, CVD, or another deposition method. ILD 48 may also be formed ofan oxygen-containing dielectric material, which may be silicon-oxidebased material such as Tetra Ethyl Ortho Silicate (TEOS) oxide,Plasma-Enhanced CVD (PECVD) oxide (SiO₂), Phospho-Silicate Glass (PSG),Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), orthe like. A planarization process such as Chemical Mechanical Polish(CMP) process or mechanical grinding process may be performed to levelthe top surfaces of ILD 48, dummy gate stacks 30, and gate spacers 38with each other.

Next, dummy gate stacks 30, which include hard mask layers 36, dummygate electrodes 34 and dummy gate dielectrics 32, are replaced withreplacement gate stacks, which include metal gates and replacement gatedielectrics as shown in FIG. 7. The respective step is illustrated asstep 210 in the process flow 200 as shown in FIG. 38. When formingreplacing gate stacks, the hard mask layers 36, dummy gate electrodes34, and dummy gate dielectrics 32 as shown in FIG. 6 are first removedin a plurality of etching steps, resulting in the trenches/openings tobe formed between gate spacers 38. The top surfaces and the sidewalls ofprotruding semiconductor fins 24′ are exposed to the trenches. Next, asshown in FIG. 7, (replacement) gate dielectric layer 52 is formed. Gateelectrode 58 is formed over gate dielectric layer 52. In accordance withsome embodiments of the present disclosure, gate dielectric 52 includesInterfacial Layer (IL) 54 (FIG. 20) as its lower part. IL 54 is formedon the exposed surfaces of protruding fins 24′. IL 54 may include anoxide layer such as a silicon oxide layer, which is formed through athermal oxidation of protruding fins 24′, a chemical oxidation process,or a deposition process. Gate dielectric 52 may also include high-kdielectric layer 56 (FIG. 20) formed over IL 54. High-k dielectric layer56 includes a high-k dielectric material such as hafnium oxide,lanthanum oxide, aluminum oxide, zirconium oxide, silicon nitride, orthe like. The dielectric constant (k-value) of the high-k dielectricmaterial is higher than 3.9, and may be higher than about 7.0. High-kdielectric layer 56 is formed as a conformal layer, and extends on thesidewalls of protruding fins 24′ and the sidewalls of gate spacers 38.In accordance with some embodiments of the present disclosure, high-kdielectric layer 56 is formed using ALD or CVD.

Gate electrode 58 may include a plurality of stacked conductivesub-layers. The sub-layers in gate electrode 58 are not shownseparately, while in the actual structure, the sub-layers aredistinguishable from each other. The formation of gate electrode 58 maybe performed using a conformal deposition method such as ALD or CVD, sothat the thickness of the vertical portions and the thickness of thehorizontal portions of the lower sub-layers of gate electrode 58 aresubstantially equal to each other.

Gate electrode 58 may include a diffusion barrier layer and one (ormore) work-function layer (not shown separately) over the diffusionbarrier layer. The diffusion barrier layer may be formed of titaniumnitride (TiN), which may (or may not) be doped with silicon. Thework-function layer determines the work function of the gate, andincludes at least one layer, or a plurality of layers formed ofdifferent materials. The material of the work-function layer is selectedaccording to whether the respective FinFET is an n-type FinFET or ap-type FinFET. For example, when the FinFET is an n-type FinFET, thework-function layer may include a TaN layer and a titanium aluminum(TiAl) layer over the TaN layer. When the FinFET is a p-type FinFET, thework-function layer may include a TaN layer, a TiN layer over the TaNlayer, and a TiAl layer over the TiN layer. After the deposition of thework-function layer(s), a barrier layer, which may be another TiN layer,is formed.

Gate electrode 58 may also include a filling metal filling all of theremaining trenches unfilled by the underlying sub-layers. The fillingmetal may be formed of tungsten or cobalt, for example. After theformation of the filling material, a planarization process such as a CMPprocess or a mechanical grinding process is performed, so that theportions of layers 52 and 58 over ILD 48 are removed. The remainingportion of gate dielectric 52 and gate electrode 58 are referred to asreplacement gate 62 hereinafter. As shown in FIG. 7, the top surfaces ofgate electrode 58, gate spacers 38, CESL 46, and ILD 48 may besubstantially coplanar at this time.

FIG. 8 illustrates the formation of self-aligned hard masks 64 inaccordance with some embodiments. The self-aligned hard masks 64 areself-aligned to the underlying replacement gates 62, and are formed of adielectric material such as ZrO₂, Al₂O₃, SiN, SiON, SiCN, SiO₂, or thelike. The formation process may include etching replacement gates 62 toform recesses, filling the dielectric material into the recesses, andperforming a planarization process to remove excess portions of thedielectric material.

In accordance with some embodiments of the present disclosure, if airgaps 39 have not been formed in preceding process steps, air gaps 39 maybe formed after the formation of replacement gates 62 and hard masks 64.The processes for forming the corresponding air gaps 39 are shown indetail in FIGS. 20 through 25 and discussed accordingly. In accordancewith alternative embodiments of the present disclosure, air gaps 39 arenot formed in gate spacers 38 at this stage. Rather, air gaps 39 areformed in subsequent processes. Accordingly air gaps 39 are illustratedas dashed to indicate they may or may not exist at this stage.

FIG. 9 illustrates the formation of source/drain contact plugs 66A andsource/drain silicide regions 69. The respective step is illustrated asstep 212 in the process flow 200 as shown in FIG. 38. The formation ofsource/drain contact plugs 66A may include etching ILD 48 and theunderlying portions of CESL 46 to form contacting openings, silicidingthe exposed portions of source/drain regions 42 to form silicide regions69, filling the contact openings with a conductive layer(s), andperforming a planarization. The resulting contact plugs 66A may includea conductive barrier layer formed of titanium, titanium nitride,tantalum, tantalum nitride, or the like, and a metal such as tungsten orcobalt over the conductive barrier layer. Source/drain contact plugs 66Amay also be formed after a dielectric layer(s) (not shown) is formedover the structure in FIG. 9, and may include portions extending intothe overlying dielectric layer(s).

In accordance with some embodiments of the present disclosure, if airgaps 39 have not been formed in preceding process steps, air gaps 39 maybe formed after the formation of source/drain contact plugs 66A. Theprocesses for forming the corresponding air gaps 39 are shown in detailin FIGS. 29 through 34 and discussed accordingly.

FIG. 36 illustrates a top view of some parts of the FinFET as shown inFIG. 9. In accordance with some embodiments of the present disclosure, aplurality of semiconductor fins 24′ are laid out as parallel strips. Aplurality of replacement gates 62 are formed as parallel strips crossingsemiconductor fins 24′. Inner sidewall spacers 37A, which are parts ofgate spacers 38, form rings encircling and contacting the sidewalls ofreplacement gates 62. Air gaps 39 also form rings encircling innersidewall spacers 37A. The sidewall portions of CESL 46 form ringsencircling air gaps 39. In accordance with some embodiments of thepresent disclosure, air gaps 39 are fully enclosed in another dielectriclayer (for example, sealing layer 76 as shown in FIGS. 19A and 19B),which also form parts of gate spacers 38. Accordingly, inner sidewallspacers 37A and the sidewall portions of CESL 46 are spaced apart fromair gaps 39 by the dielectric layer such as 76. In accordance withalternative embodiments of the present disclosure, air gaps 39 areexposed to either one, or both, of inner sidewall spacers 37A and thesidewall portions of CESL 46.

FIG. 36 illustrates two planes, with one containing line A-A, and theother containing line B-B. The plane containing line A-A is obtainedbetween two neighboring fins 24′, and hence fins 24′ (and source/drainepitaxy regions 41/42) are not in the plane. The plane containing lineB-B are obtained cutting through fin 24′, and hence fin 24′ (andsource/drain epitaxy regions 41/42) are in the plane. In the followingfigures, all cross-sectional views with STI regions 22 illustrated areobtained from the plane containing line A-A in FIG. 36, and allcross-sectional views without STI regions 22 illustrated are obtainedfrom the plane containing line B-B in FIG. 36.

FIGS. 10 through 18, 19A, and 19B are cross-sectional views ofintermediate stages in the formation of air gaps before source/drainepitaxy in accordance with some embodiments. Line 24A is illustratedthroughout the cross-sectional views to indicate the top position ofsemiconductor fins 24′. The steps shown in FIG. 10 through 17 correspondto the step shown in FIG. 3. The steps shown in FIGS. 18A and 18Bcorrespond to the steps shown in FIGS. 4, 5A, and 5B. The steps shown inFIGS. 19A and 19B correspond to the step shown in FIG. 6.

FIG. 10 illustrates dummy gate stack 30, which includes dummy gatedielectric 32, dummy gate electrode 34, and dummy mask 36. Gate spacers37 are formed on the sidewalls of dummy gate stack 30. Gate spacers 37is alternatively referred to as a dummy gate spacer since parts 37B ofgate spacers 37 will be removed. The cross-sectional view shown in FIG.10 is obtained from a plane same as the plane containing line A-A inFIG. 36. Since semiconductor fin 24′ is not in the plane, semiconductorfin 24′ is shown as dashed in FIG. 10. In the cross-sectional view, thelevel of the top surfaces 24A of fins 24′ are illustrated, andsemiconductor fins 24′ are at the level between 24A and the top surfacesof STI regions 22. Gate spacers 37 include inner sidewallspacers/portions 37A, and dummy spacer portions 37B. Portions 37A and37B are formed of different materials, which may be selected from SiO₂,SiN, SiCN, SiON, SiOCN, SiC, or the like. When formed of SiOCN, innersidewall spacers 37A may have a carbon atomic percentage greater thanabout 10 percent, and dummy spacer portions 37B may have a carbon atomicpercentage lower than about 10 percent. Dummy spacer portions 37B mayalso be formed of TiN or the composite layer of SiO₂ and SiOCN. Dummyspacer portions 37B may overlap the horizontal legs of inner sidewallspacers 37A. The thickness of sidewall portions 37A may be in the rangebetween about 2 nm and about 5 nm. The thickness of dummy spacerportions 37B may be in the range between about 2 nm and about 6 nm.

FIG. 11 illustrates the formation of buffer dielectric layer 70 andsacrificial filling material 72. In accordance with some embodiments,buffer dielectric layer 70 is formed of a material different from thematerials of portions 37A and 37B. A conformal deposition method such asAtomic Layer Deposition (ALD) or CVD may be used for depositing eitherone of dielectrics 70 and 72. Buffer dielectric layer 70 and sacrificialfilling material 72 may also be formed of SiO₂, SiN, SiCN, SiON, SiOCN,SiC, or the like. A planarization process such as a CMP process or amechanical grinding process is performed, and buffer dielectric layer 70may be used as a CMP stop layer. Furthermore, filling material 72 areformed of a material different from the materials of hard mask 36, gatespacers 37, and buffer dielectric layer 70.

Next, referring to FIG. 12, sacrificial filling material 72 is etchedback. The level of the top surface of sacrificial filling material 72partially determines the height of the subsequently formed air gaps 39(FIG. 15). Some portions of buffer dielectric layer 70 are exposed. Theexposed portions of buffer dielectric layer 70 are then etched,revealing the top edges of inner sidewall spacers 37A and dummy spacerportions 37B, as shown in FIG. 13. In the etching, the etchingselectivity values, which is the ratio of the etching rate of fillingmaterial 72 to the etching rates of materials of hard mask 36, gatespacers 37, and buffer dielectric layer 70 are high, for example, higherthan about 60, and may be higher than about 100. Accordingly, hard mask36, gate spacers 37, and buffer dielectric layer 70 are not etched.

The dummy spacer portions 37B of gate spacers 37 are then removed in anetching step. The etching may be performed using dry etching or wetetching. The etchant is selected not to attack the exposed features 37A,70, 72, 36, and possibly 34. Accordingly, trenches 74 are formed, asshown in FIG. 14. Trenches 74, when viewed in a top view, will be a ringencircling dummy gate 30 and the inner sidewall spacer 37A. Inaccordance with some embodiments, dummy spacer portions comprise siliconoxide, and are etched using the combination of NF₃, HF, and water (H₂O).Other etchant such as SiCONi (NF₃ and NH₃), Certas (HF and NH₃), and HFmay also be used. In the formation of trenches 74, the etchant isselected so that hard masks 36, inner sidewall spacers 37A, bufferdielectric layer 70, and filling material 72 are not damaged.

FIG. 15 illustrates the formation of dielectric sealing layer 76.Dielectric sealing layer 76 may be formed of SiOC or silicon nitride,for example, and other dielectric material different from the materialsof buffer dielectric layer 70 and sacrificial filling material 72 mayalso be used. In accordance with some embodiments, the formation ofdielectric sealing layer 76 includes a conformal deposition process suchas ALD or CVD, followed by a non-conformal deposition process such asPECVD. The conformal process results in a conformal layer to bedeposited on the sidewalls of inner sidewall spacer 37A and thesidewalls of buffer dielectric layer 70, with air gap 39 therebetween.The non-conformal deposition process seals the top portion of the trench74 (FIG. 14). Accordingly, air gaps 39 are formed, and are enclosed bydielectric sealing layer 76. In accordance with some embodiments of thepresent disclosure, air gaps 39 have heights H1 greater than about 75nm, and may be in the range between about 75 nm and 500 nm, depending onthe height of dummy gate 34. The width W1 of air gaps 39 may be in therange between about 2 nm and about 5 nm.

In FIG. 15 and subsequent FIGS. 19A, 19B, 26A, 26B, 35A, and 35B, theshapes of air gaps 39 are schematically shown as having rectangularcross-sectional views. It is appreciated that when the surroundingdielectric materials such as sealing layer 76 are formed using differentmaterials and/or methods, air gaps 39 as in these figures may havedifferent shapes. FIG. 37 illustrates the possible shapes of air gaps 39in accordance with some embodiments. For example, air gaps 39 may havethe shape of water droplet, ellipse shape, rectangular shape withcorners rounded, etc.

In accordance with some embodiments, the sidewall portion of sealinglayer 76 has thicknesses T1 and T1′, which may be in the range betweenabout 1 nm and about 3 nm. The ratios T1/W2 and T1′/W2 may be in therange between about 0.3 and about 0.5, wherein W2 is the width oftrenches 74 (FIG. 14).

An anisotropic etch is then performed on dielectric sealing layer 76,and the resulting structure is shown in FIG. 16. In accordance with someembodiments of the present disclosure, the etching includes a dryetching process performed using process gases such as CF₄/O₂/N₂ (themixture of CF₄, O₂, and N₂), NF₃/O₂, SF₆, SF₆/O₂, or the like. Thehorizontal portions of dielectric sealing layer 76 are thus removed.Some vertical portions of dielectric sealing layer 76 remain to seal airgaps 39. Throughout the description, inner sidewall spacers 37A,dielectric sealing layer 76, and air gaps 39 are in combination referredto gate spacers 38.

Next, the remaining portions of buffer dielectric layer 70 andsacrificial filling material 72 are removed in etching steps. Theresulting structure is shown in FIG. 17. The sidewalls of dielectricsealing layer 76 are exposed. Also, semiconductor fins 24′ are revealedagain. FIG. 17 also illustrates a cross-sectional view of the structureshown in FIG. 3. In the etching, due to the difference between bufferdielectric layer 70 and layers 70 and 72 and the selection of anappropriate etching chemical, the etching selectivity values are high,so that dielectric sealing layer 76 are not damaged. The etching may bewet etching or dry etching. The etching selectivity values are theratios of the etching rate of layers 70 and 72 to the etching rate ofdielectric sealing layer 76. For example, the etching ratios are higherthan about 100 in accordance with some embodiments of the presentdisclosure.

In subsequent steps, source/drain regions 42 are formed, and FIG. 18Aillustrates a cross-sectional view of source/drain regions 42. Thecross-sectional view shown in FIG. 18A is obtained from a plane same asthe plane containing line B-B in FIG. 36. The formation of source/drainregions 42 are also discussed referring to FIGS. 4, 5A, and 5B.

FIG. 18B illustrates the cross-sectional view obtained from the planecontaining line A-A in FIG. 36 after the formation of source/drainregions 42. Since source/drain regions 42 are not in the plane, they arenot illustrated. Instead, STI region 22 is in this plane, and is shownin FIG. 18B.

FIGS. 19A and 19B illustrate the formation of CESL 46 and ILD 48,followed by a planarization process to level the top surface of CESL 46and ILD 48 with the top surface of gate spacers 38. FIGS. 19A and 19Bshow the different cross-sectional views of the same step. FIG. 19Aillustrates the cross-sectional view obtained from the plane thatcontains line A-A in FIG. 36. FIG. 19B illustrates the cross-sectionalview obtained from the plane that contains line B-B in FIG. 36. Theperspective view of this structure is shown in FIG. 6.

FIGS. 20 through 25 illustrate the intermediate stages in the formationof air gaps 39 (FIG. 25) after the formation of replacement gates andbefore the formation of source/drain contact plugs in accordance withsome embodiments. In accordance with these embodiments, the processesshown in FIGS. 1 through 7 are performed, in which the steps shown inFIGS. 10 through 19A/19B are skipped. Accordingly, in FIG. 7, air gaps39 have not been formed yet, and will be formed in the processes shownin FIGS. 20 through 25, discussed as follows.

The cross-sectional view of the structure shown in FIG. 7 is alsoillustrated in FIG. 20. FIG. 20 illustrates gate dielectric 52, whichincludes interfacial layer 54 and high-k dielectric layer 56. Gateelectrode 58 and gate dielectric 52 in combination form replacement gate62. Hard mask 64 overlaps replacement gate 62.

Next, referring to FIG. 21, ILD 48 is recessed in an etching step,forming recesses 78. Next, protection layer 77 is formed, as shown inFIG. 22. Protection layer 77 is formed of a material different from thematerial of dummy spacer portions 37B, and different from the materialof ILD 48. In accordance with some embodiments of the presentdisclosure, protection layer 77 is formed of SiOC or another type ofdielectric material such as SiN, SiCN, or SiC. A planarization processis then performed to form the structure shown in FIG. 23, and the topedges of dummy spacer portions 37B are revealed. The resultingprotection layer 77 may have a thickness T2 in the range between about 5nm and about 10 nm. In accordance with some embodiments of the presentdisclosure. Furthermore, the ratio T2/H2 may be in the range betweenabout 0.05 and about 0.1 in accordance with some embodiments.

Next, dummy spacer portions 37B are etched, forming trenches 74 as shownin FIG. 24. The etching process may be similar to what are discussedreferring to FIG. 14, and hence is not repeated herein. Trenches 74 alsoform rings encircling the corresponding replacement gates 62. In asubsequent process, dielectric sealing layer 76 is formed, as shown inFIG. 25. In accordance with some embodiments of the present disclosure,dielectric sealing layer 76 is formed using a non-conformal depositionmethod such as PECVD, so that air gaps 39 are sealed therein. It isappreciated that air gaps 39 may have different shapes than illustrated.For example, air gaps 39 may have the shapes shown in FIG. 37 or othershapes.

In accordance with some embodiments of the present disclosure, the topsurfaces and sidewall surfaces of spacers 37A and CESL 46 have at leastsome portions exposed to air gaps 39. Dielectric sealing layer 76 mayalso partially or fully cover the top surfaces and the sidewall surfacesof inner sidewall spacers 37A and CESL 46, so that air gaps 39 are fullyenclosed in dielectric sealing layer 76, similar to what are shown inFIGS. 19A and 19B. The corresponding deposition of dielectric sealinglayer 76 may include a conformal deposition process (such as ALD orCVD), followed by a non-conformal deposition process such as PECVD.Alternatively, dielectric sealing layer 76 is formed in a non-conformaldeposition process. A planarization process is then performed. The topsurface of the remaining dielectric sealing layer 76 may be higher thanor level with the top surfaces of inner sidewall spacers 37A. Thestructure shown in FIG. 25 also corresponds to the structure shown inFIG. 8. Throughout the description, inner sidewall spacers 37A,dielectric sealing layer 76, and air gaps 39 in combination form gatespacers 38.

FIGS. 26A and 26B illustrate the formation of ILD 80, source/draincontact plugs 66A, and gate contact plugs 66B, followed by aplanarization process. FIGS. 26A and 26B show the differentcross-sectional views of the same step. FIG. 26A illustrates thecross-sectional view obtained from the plane that contains line A-A inFIG. 36. FIG. 26B illustrates the cross-sectional view obtained from theplane that contains line B-B in FIG. 36. The perspective view of thisstructure is shown in FIG. 9, except that gate contact plugs 66B are notillustrated in FIGS. 26A and 26B. Source/drain contact plugs 66A, asshown in FIG. 26B, are in contact with the source/drain silicide regions69, which are formed on the surfaces of source/drain regions 42. Inaccordance with some embodiments of the present disclosure, contactplugs 66A and 66B include a metal nitride layer (such as TiN layer) anda metal region (formed of tungsten or cobalt) over the metal nitridelayer.

FIGS. 27 through 34 illustrate the intermediate stages in the formationof air gaps 39 (FIG. 25) after the formation of both replacement gatesand source/drain contact plugs in accordance with some embodiments. Inaccordance with these embodiments, the processes shown in FIGS. 1through 9 are performed, and the steps shown in FIGS. 10 through 18 andthe process steps in FIGS. 20 through 25 were skipped. Accordingly, inFIG. 9, air gaps 39 have not been formed yet, and the processes shown inFIGS. 27 through 34 are then performed to form air gaps 39.

FIG. 27 illustrates a cross-sectional view of the structure shown inFIG. 9, in which replacement gates 62, hard masks 64, and source/draincontact plugs 66A are illustrated. Gate contact plugs have not beenformed yet. In accordance with these embodiments, as shown in FIG. 27,source/drain contact plugs 66A are shown as extending laterally tocontact opposite vertical portions of CESL 46. Accordingly, in theillustrated plane, the portions of ILD 48 between two neighboringreplacement gates 62 are all replaced with source/drain contact plugs66A. In accordance with other embodiments of the present disclosure,there may be remaining ILD 48 left on opposite sides of contact plug66A, similar to what are shown in FIG. 26A. Hard masks 64 coverreplacement gates 62. Hard masks 64 may be formed of ZrO₂, Al₂O₃, SiN,SiON, SiCN, SiO₂, or the like.

In accordance with some embodiments of the present disclosure, contactplugs 66A comprise cobalt. Protection layer 86 may be formed to protectthe underlying cobalt-containing contact plugs 66A, as shown in FIG. 28.In accordance with some embodiments of the present disclosure,protection layer 86 is a conductive layer, which may further be a metallayer formed of a metal different from the metal in contact plugs 66A.For example, protection layer 86 may be a tungsten layer. Protectionlayer 86 may also be formed of a dielectric material such as SiN, SiO₂,SiCN, or the like. The formation of protection layer 86 may includerecessing contact plugs 66A to form recess 85, as shown in FIG. 28,depositing protection layer 86 to a level higher than the top surfacesof hard masks 64, planarizing the top surface of protection layer 86,and performing an etching process to recess protection layer 86. Therespective step is illustrated as step 214 in the process flow 200 asshown in FIG. 38. The level of the top surface of protection layer 86partially determines the height of the subsequently formed air gaps. Forexample, the top surface of protection layer 86 may be level with orlower than the top surface of gate electrodes 62. Recess 85 has depthD1, which may be in the range between about 5 nm and about 10 nm. RatioD1/H3 may be in the range between about 0.08 and about 0.16, wherein H3is the height (shown in FIG. 27) of contact plug 66A before theformation and the recessing of protection layer 86.

Next, as shown in FIG. 29, CESL 46 and dummy spacer portions 37B areetched, so that their top surfaces are lowered. The respective step isillustrated as step 216 in the process flow 200 as shown in FIG. 38.Hard masks 64 and protection layer 86 protect the underlying featuresfrom being etched. Dummy spacer portions 37B are then removed in anetching step, forming trenches 74, as shown in FIG. 30. The respectivestep is illustrated as step 218 in the process flow 200 as shown in FIG.38. The etching process may be an isotropic etching process, which maybe a dry etching process or a wet etching process.

During the etching of CESL 46 and dummy spacer portions 37B, since someportions of ILD 48 (not shown in FIG. 30, refer to FIG. 9) may beexposed, an additional protection layer (not shown) may be formed beforethe etching of dummy spacer portions 37B to protect the exposed portionof ILD 48. The formation process and the material of the additionalprotection layer may be similar to that of protection layer 77 as shownin FIGS. 22 and 23. The additional protection layer protects ILD 48 frombeing etched when trenches 74 are formed.

In a subsequent step, as shown in FIG. 31, dielectric layer(s) areformed to seal air gaps 39. The respective step is illustrated as step220 in the process flow 200 as shown in FIG. 38. In accordance with someembodiments of the present disclosure, dielectric liner 87 is depositedas a conformal layer to extend into trenches 74 (FIG. 30). In accordancewith some embodiments, the sidewall portion of dielectric liner 87 hasthicknesses T1 and T1′, which may be in the range between about 0.5 nmand about 2 nm. The ratios TI/W2 and T1′/W2 may be in the range betweenabout 0.2 and about 0.4, wherein W2 is the width of trenches 74 (FIG.30). In accordance with alternative embodiments, the formation ofdielectric liner 87 is skipped. Dielectric sealing layer 76 is thenformed to seal air gaps 39. It is appreciated that air gaps 39 may havedifferent shapes than illustrated. For example, air gaps 39 may have theshapes shown in FIG. 37 or other shapes. Dielectric sealing layer 76 maybe formed using a non-conformal deposition method such as PECVD, so thatthe height of Dielectric sealing layer 76 is as great as possible, forexample, greater than about 2 nm, and may be in the range between about1 nm and about 3 nm. Protection layer 88 may then be formed. Inaccordance with some embodiments of the present disclosure, protectionlayer 88 is formed using a material, with protection layer 88 and ILD 48(FIG. 35B, not shown in FIG. 31) having a high etching selectivity. Inaccordance with alternative embodiments, depending on the materials inthe wafer, protection layer 88 may not be formed.

In a subsequent step, an anisotropic etching process (or a plurality ofanisotropic etching processes) is performed, and the horizontal portionsof layers 87, 76, and 88 are removed. The resulting structure is shownin FIG. 32. Protection layer 86 is thus revealed. The respective step isillustrated as step 222 in the process flow 200 as shown in FIG. 38.Throughout the description, the combined regions including layers 87,76, 88, 37A, and air gap 39 are in combination referred to as gatespacers 38.

FIG. 33 illustrates the formation of self-aligned dielectric regions 90.The respective step is illustrated as step 224 in the process flow 200as shown in FIG. 38. In accordance with some embodiments, the formationof dielectric regions 90 includes depositing a dielectric material, andperforming a planarization to remove excess portions of the dielectricmaterial over the top surfaces of hard masks 64.

FIG. 34 illustrates the formation of etch stop layer 92 and ILD 94.FIGS. 35A and 35B illustrate different cross-sectional views in theformation of source/drain contact plugs 96A and gate contact plugs 66B,followed by a planarization process. FIG. 35A illustrates thecross-sectional view obtained from the plane that contains line A-A inFIG. 36. FIG. 35B illustrates the cross-sectional view obtained from theplane that contains line B-B in FIG. 36. Source/drain contact plugs 96,as shown in FIG. 35B, contact the underlying protection layer 86 whenprotection layer 86 is conductive, or penetrate through protection layer86 to be in contact with contact plugs 66A when protection layer 86 isformed of a dielectric material.

The embodiments of the present disclosure have some advantageousfeatures. Since air gaps have a k value equal to 1.0, which is lowerthan the k values of other dielectric materials, the k values of thegate spacers are reduced when air gaps are included. The reduction ofthe k value of the gate spacers result in the reduction of the parasiticcapacitance between gate electrodes and nearby regions such assource/drain regions and source/drain contacts. Accordingly, the speedof the resulting FinFET is improved.

In accordance with some embodiments of the present disclosure, a methodincludes forming a gate stack over a semiconductor region, and forming afirst gate spacer on a sidewall of the gate stack. The first gate spacerincludes an inner sidewall spacer, and a dummy spacer portion on anouter side of the inner sidewall spacer. The method further includesremoving the dummy spacer portion to form a trench, and forming adielectric layer to seal a portion of the trench as an air gap. The airgap and the inner sidewall spacer in combination form a second gatespacer. A source/drain region is formed to have a portion on an outerside of the second gate spacer. In an embodiment, the method furtherincludes forming a buffer dielectric layer contacting a sidewall of thefirst gate spacer; forming a sacrificial layer over a bottom portion ofthe buffer dielectric layer; recessing the buffer dielectric layer andthe sacrificial layer to expose the dummy spacer portion, wherein thedummy spacer portion is removed after the buffer dielectric layer andthe sacrificial layer are recessed; and removing the buffer dielectriclayer and the sacrificial layer. In an embodiment, the method furtherincludes forming a contact etch stop layer contacting a sidewall of thefirst gate spacer; and forming an inter-layer dielectric over a bottomportion of the contact etch stop layer, wherein the dummy spacer portionis removed after the inter-layer dielectric is formed, and the contactetch stop layer has a sidewall exposed to the air gap. In an embodiment,the method further includes forming a first source/drain contact plugover and electrically coupling to the source/drain region; recessing thefirst source/drain contact plug to form a recess; and forming aprotection layer in the recess, wherein the dummy spacer portion isremoved w the protection layer protecting the first source/drain contactplug. In an embodiment, the protection layer is formed of a conductivematerial, and the forming the protection layer comprises forming asecond source/drain contact plug over and contacting the protectionlayer. In an embodiment, the method further includes forming adielectric liner to fill a portion of the trench, wherein the air gap isspaced apart from the inner sidewall spacer by a part of the dielectricliner. In an embodiment, the air gap has a portion extending lower thana top surface of the source/drain region.

In accordance with some embodiments of the present disclosure, a methodincludes forming a dummy gate stack on a semiconductor fin; forming adummy gate spacer on a sidewall of the dummy gate stack; forming acontact etch stop layer over a source/drain region, wherein thesource/drain region is on a side of the dummy gate spacer; forming aninter-layer dielectric over a portion of the contact etch stop layer;replacing the dummy gate stack with a replacement gate stack; forming afirst contact plug over and electrically coupling to the source/drainregion, wherein the first contact plug penetrates through the contactetch stop layer; etching a portion of the dummy gate spacer to form atrench; and forming a sealing layer to fill a top portion of the trench,wherein a lower portion of the trench is sealed as an air gap. In anembodiment, the dummy gate spacer comprises: an inner sidewall portion;and a dummy spacer portion on an outside of the inner sidewall portion,wherein the dummy spacer portion is etched to form the trench, and theinner sidewall portion remains after the etching. In an embodiment, theinner sidewall portion comprises a vertical leg on the dummy gate stack;and a horizontal leg, wherein the dummy spacer portion overlaps andcontacts the horizontal leg, and wherein the trench extends to a topsurface of the horizontal leg. In an embodiment, the method furtherincludes forming a dielectric liner to fill outer portions of thetrench, with an inner portion of the trench left as the air gap. In anembodiment, the method further includes forming a second contact plugover and contacting the first contact plug, wherein after the secondcontact plug is formed, the air gap remains. In an embodiment, themethod further includes before the portion of the dummy gate spacer isetched to form the trench, recessing the first contact plug to form arecess; and forming a protection layer in the recess, wherein when theportion of the dummy gate spacer is etched, the protection layerprotects the first contact plug. In an embodiment, the first contactplug comprises cobalt, and the protection layer comprises tungsten.

In accordance with some embodiments of the present disclosure, a deviceincludes a gate stack; a gate spacer on a sidewall of the gate stack,wherein the gate spacer comprises: an inner sidewall spacer having avertical portion contacting the gate stack; and an air gap, wherein thegate stack and the air gap are on outer side of the vertical portion ofthe inner sidewall spacer; and a contact etch stop layer having avertical portion, wherein the vertical portion of the contact etch stoplayer and the vertical portion of the inner sidewall spacer are onopposite sides of the air gap. In an embodiment, the air gap forms aring encircling the gate stack. In an embodiment, the device furtherincludes a semiconductor fin, wherein the gate stack is on sidewalls anda top surface of the semiconductor fin, and the air gap extends lowerthan a top surface of the semiconductor fin. In an embodiment, the innersidewall spacer further comprises a horizontal portion, and the air gapoverlaps the horizontal portion. In an embodiment, the contact etch stoplayer is exposed to the air gap. The device of claim 15 further includesa dielectric layer having a portion spacing the contact etch stop layerapart from the air gap.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

1. A method comprising: forming a gate stack over a semiconductorregion; forming a first gate spacer on a sidewall of the gate stack,wherein the first gate spacer comprises: an inner sidewall spacer; and adummy spacer portion on an outer side of the inner sidewall spacer;removing the dummy spacer portion to form a trench; forming a dielectriclayer to seal a portion of the trench as an air gap, wherein the air gapand the inner sidewall spacer in combination form a second gate spacer;and forming a source/drain region, wherein the source/drain region andthe gate stack are adjacent to each other.
 2. The method of claim 1further comprising: forming a buffer dielectric layer contacting asidewall of the first gate spacer; forming a sacrificial layer over abottom portion of the buffer dielectric layer; recessing the bufferdielectric layer and the sacrificial layer to expose the dummy spacerportion, wherein the dummy spacer portion is removed after the bufferdielectric layer and the sacrificial layer are recessed; and removingthe buffer dielectric layer and the sacrificial layer.
 3. The method ofclaim 1 further comprising: forming a contact etch stop layer contactinga sidewall of the first gate spacer; and forming an inter-layerdielectric over a bottom portion of the contact etch stop layer, whereinthe dummy spacer portion is removed after the inter-layer dielectric isformed, and the contact etch stop layer has a sidewall exposed to theair gap.
 4. The method of claim 1 further comprising: forming a firstsource/drain contact plug over and electrically coupling to thesource/drain region; recessing the first source/drain contact plug toform a recess; and forming a protection layer in the recess, wherein thedummy spacer portion is removed w the protection layer protecting thefirst source/drain contact plug.
 5. The method of claim 4, wherein theprotection layer is formed of a conductive material, and the forming theprotection layer comprises forming a second source/drain contact plugover and contacting the protection layer.
 6. The method of claim 4further comprising forming a dielectric liner to fill a portion of thetrench, wherein the air gap is spaced apart from the inner sidewallspacer by a part of the dielectric liner.
 7. The method of claim 4,wherein the air gap has a portion extending lower than a top surface ofthe source/drain region.
 8. A method comprising: forming a dummy gatestack on a semiconductor fin; forming a dummy gate spacer on a sidewallof the dummy gate stack; forming a contact etch stop layer over asource/drain region, wherein the source/drain region is on a side of thedummy gate spacer; forming an inter-layer dielectric over a portion ofthe contact etch stop layer; replacing the dummy gate stack with areplacement gate stack; forming a first contact plug over andelectrically coupling to the source/drain region, wherein the firstcontact plug penetrates through the contact etch stop layer; etching aportion of the dummy gate spacer to form a trench; and forming a sealinglayer to fill a top portion of the trench, wherein a lower portion ofthe trench is sealed as an air gap.
 9. The method of claim 8, whereinthe dummy gate spacer comprises: an inner sidewall portion; and a dummyspacer portion on an outside of the inner sidewall portion, wherein thedummy spacer portion is etched to form the trench, and the innersidewall portion remains after the etching.
 10. The method of claim 9,wherein the inner sidewall portion comprises: a vertical leg on thedummy gate stack; and a horizontal leg, wherein the dummy spacer portionoverlaps and contacts the horizontal leg, and wherein the trench extendsto a top surface of the horizontal leg.
 11. The method of claim 8further comprising forming a dielectric liner to fill outer portions ofthe trench, with an inner portion of the trench left as the air gap. 12.The method of claim 8 further comprising forming a second contact plugover and contacting the first contact plug, wherein after the secondcontact plug is formed, the air gap remains.
 13. The method of claim 8further comprising: before the portion of the dummy gate spacer isetched to form the trench, recessing the first contact plug to form arecess; and forming a protection layer in the recess, wherein when theportion of the dummy gate spacer is etched, the protection layerprotects the first contact plug.
 14. The method of claim 13, wherein thefirst contact plug comprises cobalt, and the protection layer comprisestungsten. 15.-20. (canceled)
 21. A method comprising: forming a gatespacer comprising: a first portion comprising a vertical leg and ahorizontal leg, wherein the vertical leg has a bottom end connecting toan end of the vertical leg; a second portion overlapping the verticalleg; forming a gate stack contacting a sidewall of the vertical leg;forming a dielectric layer contacting a sidewall of the second portionof the gate spacer; etching the second portion of the gate spacer toform a trench between the dielectric layer and the gate stack; andforming a dielectric region extending into at least a top portion of thetrench, with a lower portion of the trench unfilled to form an air gap.22. The method of claim 21 further comprising forming a shallow trenchisolation region, wherein each of the horizontal leg, the dielectriclayer, and the gate stack comprises a portion having a bottom surfacecontacting a top surface of the shallow trench isolation region.
 23. Themethod of claim 21, wherein the forming the dielectric region comprises:forming a dielectric liner extending into the trench, wherein the airgap is spaced apart from the dielectric layer and the vertical leg ofthe first portion of the gate spacer by the dielectric liner.
 24. Themethod of claim 23, wherein the forming the dielectric region furthercomprises forming a sealing layer, wherein the sealing layer has abottom surface exposed to the air gap.
 25. The method of claim 23further comprising performing a planarization process to level a topsurface of the dielectric liner with a top surface of the vertical leg.26. The method of claim 21 further comprising forming a conductive plughaving a sidewall contacting a sidewall of a vertical portion of thedielectric layer.